1. Field of the Invention
The present invention relates to a stencil sheet roll and to a method for preparing the roll. More particularly, the invention relates to a stencil sheet roll in which smoothness of the surface of a stencil sheet which has been wound up in a roll is prevented from becoming deteriorated.
2. Description of the Related Art
A stencil sheet is prepared, for example, through laminating a thermoplastic resin film to a porous support using an adhesive, and applying a mold releasing agent onto the surface of the thermoplastic resin film so as to prevent melt-sticking of film. Stencil sheet rolls are manufactured by winding up the above-mentioned stencil sheets to form rolls. Stencil sheet rolls are set in automatic stencil making/printing device without being unwound for automatic stencil making and printing.
Presently, thermal printing head (TPH) are used to make stencils for stencil printing. In this case, if the surfaces of stencil sheets are rough, uniform contact cannot be obtained between TPH and a film, causing some parts to be easily perforated, and other parts to be not. As a result, images of the original text or drawing cannot be perforated at precise locations, and satisfactory printing images cannot be obtained. To avoid this shortcoming, it has been attempted to secure surface smoothness of a stencil sheet by the use of a porous support having good surface smoothness.
Meanwhile, in present-day stencil printing, stencil sheet rolls, i.e., stencil sheets wound up in rolls, are frequently used so as to enhance working efficiency. In this case, even though the stencil sheet has excellent surface smoothness in a flat sheet state, winding pressure applied to the sheet during winding to form a roll (generally 0.1-1.2 kg/cm.sup.2) deteriorates surface smoothness as time passes. Therefore, stencil sheet rolls have an inherent drawback that they cannot maintain surface smoothness of flat stencil sheets. Smoothness of the surface of a stencil sheet roll is deteriorated because as time passes a portion of thermoplastic resin film that is not supported by fibers in a porous support gradually yields to form recesses under winding pressure of the stencil sheet which is wound in layers.
In order to solve this problem, a method has been proposed in which winding density of a roll is adjusted so as to prevent deterioration in smoothness of the surface of a stencil sheet (Japanese Patent Application Laid-Open (kokai) No. 6-239048). However, the approach of adjusting the winding density of a roll by itself is not effective in preventing deterioration of smoothness of a stencil sheet at interior portions of the roll where relatively high winding pressure is applied, particularly in the vicinity of the core of the roll. If winding pressure of the roll is reduced so as to avoid this drawback, another problem is caused in that handling of the roll becomes poorer.